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Sound Advice

70-V Audio Distribution: Part 1 by Al Whale

When required to connect speakers over large areas with non-interacting area controls like halls or classrooms, the 70-V system is ideal, although it should be noted that some places consider 70-V systems to be unsafe, so the 25-V system is used instead.
Transformers are used at each speaker location to convert from the 70(25)-V system to the speaker impedance (eg. 8 ohms).

The 70(25)-V line from the amplifier is applied to the input of the transformer. The input selected is based on the maximum power needed from the speaker. Each speaker location comes from this same 70(25) volt line source (in parallel). The sum of the power setting of all transformers used should be less than the maximum power of the amplifier.
Be warned: if the total is over the maximum setting, the amplifier will be overloaded and there will no longer be a constant output. Switching a group of speakers in this situation will then affect the other speakers.

From the above calculations, the 10-watt tap will be 500 ohms, and the 5-watt tap will be 1,000 ohms. When wiring, a smaller gauge wire can be used to go long distances without affecting the audio due to line loss.
Example: If the total load on the 70-V line is 100 watts, from the above formula, the impedance would be 50 ohms. Using the practice of 5% max, the wire would have to be under 2.5 ohms. Checking wire tables, for 50-ft. run, the wire would only need to be #22 gauge (1.614 ohms). For 500 ft., the wire would be #12 gauge (1.588 ohms). This is far easier than using 8-ohm lines – #16 & #6 gauge, respectively.

Al Whale is Broadcast Technologist and Assistant Chief Engineer at CHBC-TV. He also performs maintenance, design, and installation se-tup. He has operated and taught sound in many church settings. Visit Al’s website at: www.whalco.ca.

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